On a medium with Multiple Access, multiple nodes may concurrently send and receive. The concurrent transmission by multiple nodes may result in frame collisions, in which a receiver is unable to extract frames from overlapping received signals.
The device attempting to transmit may have a certain working bandwidth which is different from the intended receiver's working bandwidth. The overlapping area of those two bandwidths forms the common bandwidth that may be used in the current MC system.
Two different multiple access mechanisms are the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) and the CSMA/CD (Carrier Sense Multiple Access with Collision Detection). In the mechanism Collision Avoidance a data station that intends to transmit firstly sends a reservation signal, secondly after waiting a sufficient time for all stations to receive the reservation signal transmits a frame and thirdly while transmitting, if the station detects a reservation signal from another station, it stops transmitting for a random time and then reattempts. Whereas in the mechanism Collision Detection a station that detects another signal while transmitting a frame stops transmitting the frame, transmits a collision indicating signal and then waits for a random time interval before reattempting to send that frame.
In a radio network a large area can be covered if the subsets, for example clusters, in which the network is divided, are interconnected by forwarders or bridges, for example. Subscribers of two neighbouring clusters cannot directly communicate when one subscriber is not in the reception range of the other. Signals sent by a forwarder can be received by members of both neighbouring clusters. One mechanism used to minimize the hidden node problem is the on demand mechanism Request-to-Send/Clear-to-Send (RTS/CTS) of the Ethernet. Before the transmission of a data frame starts the sending station sends an RTS frame which is answered by a CTS frame of the receiving station. The RTS and the CTS frame contain information about the length of the transmission duration of the subsequent data frame and about the corresponding ACK frame (Acknowledgement). By using the RTS/CTS mechanism it is achieved that a third station as a hidden node that is too far away from the sending station to receive the RTS frame, but sufficiently close to the receiving station to receive the CTS frame will not start a transmission on its own as it realizes the observed channel as not being idle.
Between two frames in the sequence RTS, CTS, DATA and ACK a Short Interframe Space (SIFS) could be arranged which is of 16 μs duration in the case of the protocol 802.11a.
The MC-CDMA mechanism is a digital modulation technique that spreads each data symbol of the data stream in the frequency domain as shown in FIG. 1 concerning the prior art. A copy of each original symbol or chip is placed in each of the narrow band subcarriers. The total channel bandwidth is divided into subcarriers, in this example into four subcarriers f1 to f4. Assuming that the symbol duration is Tb the spectral distance of the subcarriers should be at least 1/Tb.
In the example of FIG. 1 the original signals of the 4 users are spread into 4 subcarriers, each fed with one copy of the original signal (chip) thus giving a Spreading Gain of 4. In order to differentiate between the various chips on one channel or subcarrier each chip is encoded as the used spreading code defines it. Data belonging to the same channel or subcarrier are all spread with the same binary code sequence of length L.
For example when using a frequency-shift keying
a “0” in the third place of the binary code sequence means that a factor π will be added to the phase of third chip that will be put to the 3rd subcarrier and
a “1” in the third place of the binary code sequence corresponds to a phase shift of 0 as regards the phase of the chip.
That symbol or that signal that is finally transmitted on the nth subcarrier corresponds to the sum of each of the nth chips of k parallel channels, with k=4 in this example. The variable k corresponds to the amount of active users, subscribers, terminals or stations respectively.
As different devices have different subsets of available subcarriers in which they can operate, two devices intending to initiate a connection have to find a common set of subcarriers or channels respectively on which they are going to exchange information.
Each channel uses a centre frequency of the bandwidth area that is variable. For example a channel that needs for its operation a certain number (e.g. 8) of subcarriers has to choose the subcarriers out of a group of subcarriers which are the common or overlapping subcarriers of transmitter's available subcarriers and the receiver's available subcarriers.
One object of the invention is to provide a method for signalling the status of a subcarrier in a Multi Carrier network.
A further object of the invention is to provide a method for adaptively allocating to a connection certain subcarriers of those subcarriers in a Multi Carrier network that are currently idle.